Wastewater treatment systems are widely used throughout the world to remove nutrients such as nitrogen and phosphorous from the wastewater as well as destroy pathogens and viruses within the resulting sludge before the purified effluent is discharged and before the final treated sludge is removed from the wastewater treatment system.
Typically, in a wastewater treatment system, wastewater influent is directed through a series of a secondary treatment zones and subjected to various forms of treatment such as, for example, anaerobic, aerobic, and/or anoxic treatment. After such treatment, the wastewater is directed to a final clarifier which separates sludge in the wastewater from purified effluent. The purified effluent is discharged into a stream or lake, for example, while the sludge from the final clarifier is returned to the head of the activated sludge system and mixed with the influent wastewater to form what is commonly referred to as mixed liquor. Throughout the wastewater treatment process, the sludge from the final clarifier is recycled through the activated sludge system. The biomass or microorganisms associated with the recycled sludge act to effectively remove nutrients such as nitrogen and phosphorous and reduce BOD and other contaminant levels within the wastewater being treated.
However, the sludge being recycled through the activated sludge system has to be continually removed or wasted from the process. In addition, depending on a number of factors such as the contaminant levels in the wastewater influent, certain amounts of primary sludge may be removed during primary treatment without being processed through the activated sludge system. This primary sludge is then mixed with the secondary sludge wasted from the activated sludge process and the mixture is subjected to further treatment where contaminants are removed or separated from the sludge. In a typical wastewater treatment system, this sludge mixture is directed to a digester where the sludge is treated and cleaned by removing pathogens and volatile solids. Usually, one of the most convenient methods of disposal of the resulting sludge is, for example, by land applications.
Existing wastewater facilities are often designed and built to handle a particular processing capacity and to produce a certain quality of effluent. As technology improves or as conditions change over time, such as where the quality of the wastewater deteriorates or where the processing capacity of the system is exceeded, these wastewater facilities become relatively inefficient.
In addition, many treatment processes for wastewater are batch-type processes. This means that large tanks are necessary in order to process the large batches of wastewater where these tanks are sized relative to the capacity of the treatment plant. Accordingly, the throughput of a conventional wastewater treatment system is limited by the capacity of the tanks and the time required for each batch process in the wastewater treatment procedure.
Batch processing of sludge also create a number of additional problems. For instance, treated sludge resulting from a batch process performed in a large tank may exhibit nonuniformities which may be attributed to such causes as poor mixing of treatment chemicals with the sludge or, where a heat treatment is used, to improper distribution of heat within the sludge. Excessive agitation of the sludge or extended sludge retention time may often be necessary to compensate for the shortcomings in batch processes, but these measures are not always effective. Furthermore, pipes, pumps, valves, and associated equipment used for directing the sludge through the treatment system spend a significant amount of time in a dry state after batches of sludge have been transferred to the processing tanks. Thus, any residual sludge remaining in these peripheral items will dry and leave a residue during each subsequent dry state, leading to frequent, difficult, and costly maintenance of these parts.
On the other hand, experience has shown that some of the processes in a wastewater treatment system are actually more efficient when operated in a continuous manner instead of being subject to batch processing. For example, anaerobic digesters are generally more efficient when exposed to a continuous flow of sludge. This way, the microorganisms which are operative during the anaerobic treatment are maintained in a continuous active state and are thus more effective in removing contaminants from the sludge. In contrast, in a batch process where the activity of the microorganisms tapers off near the end of the digestion process, additional time is required in a subsequent batch of sludge to rejuvenate the microorganisms to an effective level. That is, batch processing limitations again result in additional process time for treating the sludge.
The incompatibility between batch and continuous flow processes is evident particularly in a wastewater treatment system which must be expanded to meet increased processing demands. Since these plants are typically arranged to operate in a batch mode, large additional batch tanks are necessary and the physical land space required for this new equipment is often measured in terms of acres. In addition, the costs involved in such an undertaking are often relatively high when compared to the limited flexibility for future expansion gained for the investment. Furthermore, the commensurate increase in operating and maintenance costs combined with the other shortcomings suggests that a more flexible and efficient alternative is needed.